We present a comprehensive study of the crystal structure, magnetic structure, and microscopic magnetic model of (CuBr)LaNb${}_2$O${}_7$, the Br analog of the spin-gap quantum magnet (CuCl)LaNb${}_2$O${}_7$. Despite similar crystal structures and spin lattices, the magnetic behavior and even peculiarities of the atomic arrangement in the Cl and Br compounds are very different. The high-resolution x-ray and neutron data reveal a split position of Br atoms in (CuBr)LaNb${}_2$O${}_7$. This splitting originates from two possible configurations developed by [CuBr] zigzag ribbons. While the Br atoms are locally ordered in the $ab$ plane, their arrangement along the $c$ direction remains partially disordered. The predominant and energetically more favorable configuration features an additional doubling of the $c$ lattice parameter that was not observed in (CuCl)LaNb${}_2$O${}_7$. (CuBr)LaNb${}_2$O${}_7$ undergoes long-range antiferromagnetic ordering at $T_N=32$ K, which is nearly 70$\%$ of the leading exchange coupling $J_4\simeq 48$ K. The Br compound does not show any experimental signatures of low-dimensional magnetism because the underlying spin lattice is three-dimensional. The coupling along the $c$ direction is comparable to the couplings in the $ab$ plane, even though the shortest Cu–Cu distance along $c$ (11.69 Å) is three times larger than nearest-neighbor distances in the $ab$ plane (3.55 Å). The stripe antiferromagnetic long-range order featuring columns of parallel spins in the $ab$ plane and antiparallel spins along $c$ is verified experimentally and confirmed by the microscopic analysis.

• Received 27 March 2012

DOI:https://doi.org/10.1103/PhysRevB.85.214427